1. Field of the Invention
The present invention relates to a liquid crystal display (particularly to a segment display-type liquid crystal display).
2. Description of the Related Art
With a general liquid crystal display, electrodes are provided on the upper and lower substrates respectively, and alignment films are provided on the electrodes, respectively. In addition, the upper and lower substrates are superimposed so that the respective electrodes face each other. Here, in order to provide a liquid crystal film having a predetermined thickness (for instance, several μm) between the upper and lower substrates, a spherical spacer is disposed between the upper and lower substrates. Polarizers are attached to the outer sides of the upper and lower substrates, respectively. In this kind of liquid crystal display, by applying voltage to the liquid crystal film by using the respective electrodes of the upper and lower substrates, the alignment of the liquid crystal film of the display area, which is the portion that both electrodes are superimposed, is changed in order to switch the bright display state and dark display state in terms of appearance.
With a vertical alignment liquid crystal display, since the retardation is substantially zero when the liquid crystal display is viewed from the front during non-application of voltage, it is characterized in that an extremely favorable dark display state can be obtained by disposing the respective polarizers in a crossed Nicol arrangement. This vertical alignment liquid crystal display can also realize a normally black display with favorable visual angle characteristics during non-application of voltage by additionally disposing a visual angle compensating plate between the liquid crystal film and at least one of the polarizers. Moreover, an STN (Super Twisted Nematic)-type liquid crystal display in which the twist angle of the liquid crystal molecules of the liquid crystal film between the upper and lower substrates is set to about 180° to 240° has a laminated structure provided with optical compensation cells, and is characterized in that a favorable dark display state when the liquid crystal display is viewed from the front can be obtained by disposing the respective polarizers in a crossed Nicol arrangement. The term “optical compensation cells” as used herein refers to the liquid crystal cells which basically have the same structure as the foregoing STN-type liquid crystal display, and which are disposed in a manner where the twisting direction of the liquid crystal film is relatively opposite each other, and the alignment direction of the liquid crystal molecules at the substantial center of the liquid crystal film in the layer thickness direction is orthogonal. Note that the foregoing optical compensation cells may be substituted with a liquid-crystalline polymer film having similar optical characteristics. Moreover, as one type of STN-type liquid crystal display described above, also known is a film-compensated STN-type liquid crystal display in which a retardation film having positive uniaxial anisotropy is disposed between the liquid crystal film and the respective polarizers. A normally black display can be realized with any of the STN-type liquid crystal displays described above.
Moreover, known is a segment display-type liquid crystal display capable of displaying a display pattern including a predetermined design or text. This kind of segment display-type liquid crystal display comprises an effective display area including a plurality of segment display areas for displaying, for example, an arbitrary design or the like, and an external extraction electrode terminal area for electrically operating the respective display areas of the effective display area. The term “effective display area” as used herein refers to an area that is exposed without being covered when housing the liquid crystal display in a case of various devices, and which is viewable from the outside.
In order to realize the foregoing segment display-type liquid crystal display, a segment electrode and a common electrode are provided to one face of each of the upper and lower substrates, the upper and lower substrates are superimposed so that both electrodes face each other, and the area where both electrodes overlap form be a predetermined display pattern. Here, the portion where the segment electrode and the common electrode do not overlap is referred to as a “routing wire”, and functions as a wire for connecting the portion to be used for display to the external extraction electrode. The layout of this kind of routing wire needs to be designed so that the routing wire is arranged on only either one of the upper or lower substrate. This is because, when the routing wires overlap, change in the alignment of the liquid crystal film in such overlapping area, which is normally not required, will occur, and cause a display defect. A previous example of this kind of liquid crystal display is disclosed, for example, in JP-A-2006-309117 (Patent Document 1).
With a conventional liquid crystal display, as the respective upper and lower substrates, for instance, a glass substrate having a thickness of about 0.3 to 1.1 mm has been often used. This is because a glass substrate has a high glass transition point of 500° C. or higher, has superior resistance against various chemicals, has relatively superior workability and, therefore, a glass substrate can broaden the options of various chemicals to be used in high-temperature processes of 150° C. or higher and electrode patterning, and can be handled favorably. Meanwhile, a liquid crystal display is also provided which uses a flexible plastic substrate or film substrate. This kind of liquid crystal display is advantageous in that it is possible to realize a lighter weight and a thinner profile in comparison to a liquid crystal display using a glass substrate, and is also advantageous in that it is flexible and superior in shock resistance. Thus, it is possible to relatively easily realize a shockproof display device or a curved display device that is difficult to be realized when a glass substrate is used.
Nevertheless, since the photolithography technique is often used for the patterning of the segment electrode and the common electrode on the upper and lower substrates, this includes numerous processes that considerably damage the substrate; for instance, heating and cooling of the substrate, irradiation of ultraviolet rays, and exposure to acid chemicals and alkali chemicals. When using the foregoing plastic substrate or film substrate, these substrates need to comprise high-temperature process resistance, chemical resistance and handling performance that are equivalent to a glass substrate. Nevertheless, options of such a plastic substrate and the like that satisfy all of the foregoing conditions are limited, and also disadvantageous in terms of cost. In addition, since a substrate is disposed between two polarizers on application to a liquid crystal display, the phase difference in the substrate surface needs to be substantially zero, and the options are even more limited. Thus, simplification of the common electrode is desired so that the patterning of the common electrode can be omitted, or reduced as much as possible. Nevertheless, if simplification of causing the entire surface of the substrate to be the common electrode is performed, as described above, change in the alignment of the liquid crystal film will occur in the overlapping area of the routing wire connected to the segment electrode, and the common electrode, the change in the alignment being normally not required, thereby causing a display defect.
With any of various kinds of liquid crystal displays described above, on the other hand, in order to maintain the gap of the upper and lower substrates and cause the liquid crystal film to have a uniform layer thickness, a spherical spacer is disposed between the upper and lower substrates. This kind of spherical spacer is equally and randomly dispersed, in the manufacturing process of a liquid crystal display, on one of the substrates via the dry spraying method which is described, for example, in JP-A-2001-21899 (Patent Document 2). Nevertheless, with the foregoing method, since the spacer is randomly disposed on one of the substrates, the spacer may be in the display area. The spacer that is in the display area as described above will induce the non-uniformity of the alignment in the liquid crystal film during non-application of voltage or during voltage application, and may cause a drop in the display quality of the liquid crystal display.
Meanwhile, proposed is a liquid crystal display element having a structure of maintaining the gap of the upper and lower substrates by using a columnar spacer made from photosensitive resin in substitute for the spherical spacer. With a liquid crystal display having the foregoing structure, since a spacer can be intentionally disposed at a position where an alignment defect will not occur in the display area, it is possible to prevent the drop in the display quality during non-application of voltage or during voltage application. This kind of columnar spacer is mainly used in a dot matrix-type liquid crystal display in which rectangular pixels are formed in a matrix. In this case, a structure where the columnar spacer is disposed below the black mask, which is disposed outside of the effective pixels, and the columnar spacer is not disposed in the effective pixels, would be well known.
Meanwhile, the spherical spacer or columnar spacer that is generally used in a liquid crystal display as described above is formed using a material having optical isotropy, unlike a liquid crystal material having optical anisotropy. Accordingly, since the retardation between the liquid crystal film and the compensating plate cannot be canceled out unless a light-shield film is disposed on at least one of the substrates in an area where the spacer is disposed, leakage of light will occur in the area where the spacer is disposed when the liquid crystal display is viewed from the front or viewed from an oblique direction, and cause a drop in the display quality of the liquid crystal display. Note that, while it is relatively easy to provide a light-shield film in correspondence to a columnar spacer in which the positional arrangement thereof is predetermined, it is difficult to provide a light-shield film in a case of a spherical spacer that is randomly disposed on the substrate surface via the dry spraying method or the like, in accordance with the position thereof. Meanwhile, it is also possible to consider providing light shielding performance to the columnar spacer itself. Specifically, for instance, considered may be forming the columnar spacer using photosensitive resin with carbon particles dispersed therein. Nevertheless, with this type of photosensitive resin, since the photosensitivity and patterning performance tend to deteriorate as a result of increasing the film thickness, it is difficult to obtain a columnar space with sufficient light shielding performance if the film thickness is made to be thicker than 2 μm under the existing conditions. Moreover, since the number of processes will increase as a result of forming a light-shield film, a light-shield film is not being positively used in a black-and-white liquid crystal display, which does not use a color filter, since it will increase costs.